Show Hints / Candidates will help you see the available pencil values within the cell, row, column, and boxes.

Sudoku How To Technique: Jelly Fish

Taking Sword Fish to the next level by adding an additional number of valid ROWS and COLUMNS form three to four. What you need to look for is a VALID HINT that shows four or less times within a primary ROW or COLUMN that share the same four alternate ROW or COLUMNS. To be valid the HINT cannot show up more than four times within the primary ROW or COLUMN. It can show up more than four times within the alternate ROW or COLUMN or we wouldn’t have anything to eliminate.

Reviewing the example, the primary COLUMNS C1, C4, C7 and C8 have a HINT value of {5}, that share the secondary ROW CONTAINER. Because of that, we are safe to remove the HINT value of {5} anywhere else on the secondary container. Think of the primary container as “safe cells”, and you do not remove hints from those.